The discovery that methylation of homocysteine is impaired in malignant and embryonic cells but not in normal cells in tissue culture has directed our attention to the metabolism of folic acid and its derivatives. We have recently detected a cleavage system which accounts for the formation of pterin-6-carboxaldehyde as a degradation product of folic acid found only in malignant cells. In addition to the production of pterin-6-carboxaldehyde in tissue culture, we have demonstrated the high correlation between its excretion in the urine and the occurrence of malignant disease in individuals. We are currently examining methods of quantitatively determining pterin-6-carboxaldehyde by high pressure liquid chromatography and purifying from malignant tissue the enzyme system responsible for its production. BIBLIOGRAPHIC REFERENCES: R. Halpern, B. C. Halpern, B. Stea, A. Dunlap, K. Conklin, B. Clark, H. Ashe, L. Sperling, J. A. Halpern, D. Hardy, and R. A. Smith, Pterin-6-aldehyde, a cancer cell catabolite: Identification and application in diagnosis and treatment of human cancer, Proc. Natl. Acad. Sci. USA 74, 587 (1977).